Cyclic AMP and cyclic GMP have been implicated as regulatory intermediates affecting many processes including cardiac contraction. Papillary muscle preparations, isolated perfused hearts and embryonic chick hearts will be used to evaluate further the possibility that guanyl cyclase and adenyl cyclase are linked to the cholinergic receptor and the beta adrenergic receptor, respectively. Both cyclase systems will be studied in vivtro following various drug treatments with particular emphasis on whether cholinergic agents can stimulate guanyl cyclase. Since a correlation exists between increased cardiac cyclic GMP levels and the negative inotropic effect of acetylcholine (ACh), other drugs causing negative inotropism or positive inotropism will be studied in these cardiac preparations. The levels of cyclic AMP and cyclic GMP will be determined simultaneously in cardiac preparations to which various ions and other cardioactive agents have been added. Receptor blocking agents will be used to help characterize the receptors for these cardioactive agents. An attempt will be made to associate the negative-and positive-(under special conditions) inotropic effects of ACh will concomittant changes in cardiac levels of cyclic nucleotides. The effects of ACh and other agents on the hydrolysis of cyclic AMP or cyclic GMP by phosphodiesterase preparations from cardiac muscle will also be evaluated. The time course of appearance of adenyl- and guanyl-cyclase activities in the embryonic heart will be correlated with the appearance of sympathetic and parasympathetic innervations and the developing responsiveness of the hearts to autonomic agents. Effects of cyclic AMP and cyclic GMP on calcium accumulation (binding or uptake) by the cardiac sarcoplasmic reticulum or by cyclic AMP-dependent protein kinases will be studied. Cardiac miscrosomes will also be tested for the presence of a cyclic GMP-dependent protain kinase that may be associated with calcium accumulation.